DE68925345T2 - Pressed fabric with foam coating to control the empty volume - Google Patents

Abstract

This invention is directed to coating press fabrics to achieve a controlled void volume. More particularly, this invention is directed to a method of modifying a press fabric for a papermaking machine which comprises the steps of: (a) applying a thin layer of a polymeric foam to the surface of a press fabric; (b) drying said foam to form a coated press fabric; and (c) curing said coated press fabric.

Description

The present invention relates to press fabrics with a foam coating. More specifically, the present invention relates to the coating of press fabrics to achieve a controlled void volume and a controlled permeability.

It is known in the art to apply various types of layers to a papermaking press fabric. In WO 88/07929 (Wood) a process is disclosed in which at least one layer of an elastomeric foam has been applied to a papermaking press fabric to reduce the amplitude of vibration resulting from the high rotational speeds of the press rolls. The elastomeric foam may be in the form of a foam sheet which is adhesively bonded to the base fabric by needling and other disclosed techniques may be used. It is also disclosed that the elastomeric foam may be applied in situ to the base fabric by mixing a liquid polymer and air to produce a foam mixture which is applied to the base fabric. After application, the structure is heated to dry out the remaining solvents and chain extenders in the foam and to create a pore size in the foam, thereby forming the foam surface.

However, in WO 88/07929 the aim of the layer is not to achieve a controlled void volume and to improve the properties of the press fabric surface to achieve a predetermined permeability. In the document mentioned, the foam layer is applied on or near the surface of a base fabric It is also described that layers of needle punched fibrous material are placed thereon sandwiching the foam layers between them. This foam layer thus has no effect on the surface of the press fabric and this foam layer certainly does not have the purpose of controlling the void volume of the press fabric but is concerned with minimising the amplitude of vibration in the manner mentioned. Sandwiching the foam layer may achieve this aim but it certainly has no effect on the surface smoothness or finish. GB-A-2 148 377 discloses an industrial fabric, in particular a papermaking drying fabric, and a process for making the same. The process consists in introducing a liquid foaming material or foaming materials into a base structure consisting of a tie tape or a woven or knitted fabric and curing or setting the fabric to thereby form a continuous layer of elastomeric foam with open cells within the fabric. The resulting fabric shows reduced permeability, which is achieved by almost complete impregnation of the fabric.

Background of the invention

Papermaking press fabrics are rotating belts made of fibrous material used to convey a wet paper web fed from a wet papermaking machine from a forming section through a press section to a drying section. The press section normally includes rotating cylindrical nip rolls between which the freshly formed paper web is passed. As the web enters the nip of the rolls, water is squeezed out of the paper and absorbed by the press fabric on which the paper is conveyed through the nip.

Papermaking press fabrics are well known. Such fabrics are typically formed from materials such as wool, nylon and/or other synthetic polymeric materials and the like. With such fabrics, the paper web after passing through the nip of the press rolls normally still contains a significant amount of water, which results in a significant increase in manufacturing costs due to the high amount of energy required to evaporate the water during the subsequent drying step. In order to reduce manufacturing costs, it would therefore be extremely advantageous if the permeability and water removal capacity of the press fabric could be increased and/or maintained for a longer period of time. Other goals are to create a smoother surface free of pin marks; an increased paper web contact area; and an even pressure distribution.

Objectives of the invention

An object of the invention is to provide an improved press fabric.

Another object of the invention is to provide a method for treating a press fabric to achieve a predetermined permeability.

Yet another object of the invention is to provide a relatively simple and calculable method for adjusting the gap volume of a press fabric.

These and other objects of the invention will become more apparent from the following discussion.

Short description of the drawings

Fig. 1 is a graphical representation of web solids content versus fabric wrap as produced by various press fabrics, including an embodiment of the invention; and

Fig. 2 shows a graphical representation of the relationship between paper web smoothness and press load for various press fabrics including an embodiment of the invention.

Detailed description of the invention

According to the present invention, a method is provided for modifying a papermaking press fabric to adjust its permeability. More specifically, a papermaking press fabric is treated with one or more layers of polymeric foam which are dried and then cured.

The press fabrics to be modified include the press fabrics known in the art. Typical such fabrics are, for example, described in US Patent Nos. 2,354,435, 2,567,097, 3,059,312, 3,158,984, 3,425,392, 3,617,442, 3,657,068 and 4,382,987 and in British Patent No. 980,288.

It will be apparent to those skilled in the art that coating other substrates in the manner described would result in structures having adequate paper web dewatering capability. Examples of some such substrates are woven and nonwoven structures with or without needle punched fibers; composite structures consisting of multiple fiber configurations, air-laid and wet-laid fiber webs, and the like.

Useful resin compositions include synthetic, flexible, polymeric resin foams. Also useful are foams based on polyurethanes, polyethers, polyesters, polysilicon, polyacrylic, polyvinyl chloride, polyisocyanate, epoxy, polyolefins or polyacrylonitrile, rubber foam and the like. A combination of two or more such elastomeric resins may also be used. Typical useful resin compositions are Emulsion 26172 (an acrylic emulsion representative of a wide range of emulsions available from B. F. Goodrich) and Permuthane HD2004 (a water-based polyurethane emulsion available from C. L. Hauthaway).

It is understood that the resin composition may be solvent-based, water-based, high solids (i.e., with little or no solvent), or with a combination of solvents or co-solvents that results in complete or partial dissolution and/or suspension of the resin particles. This would also include water-based plastisols as well as other emulsions.

In addition, the foam may contain one or more surfactants, emulsifiers, stabilizers or the like. Examples of such additives include ammonium stearate, ACRYOL TT678 available from Rohm & Haas, ASE 60, TAMOL, TRITON surfactants, PLURONIC L62 from BASF and the like.

The foam structure in the final form can be either an open (i.e., reticulated) or closed cell structure, or a combination of both. In some cases, collapse of the foam during curing results in coating or bridging of the substrate fibers. Any of these shapes or combinations result in a change in the substrate properties.

According to the invention, a foam is applied to a surface or surfaces of a press fabric, after which it is allowed to dry and then cured. Drying and curing can be carried out in separate steps or simultaneously. In some In some cases it may be desirable to calender the fabric after the drying step and before the curing step.

The foam could be applied by any of a number of known processes, including, for example: doctor blade coating techniques which can be done on a roller, off a roller or on a table; press coating; transfer coating; spraying; sanding rollers or applicator rollers; slot applicator; and brush application. A single layer can be applied, or multiple layers of the same or different foam formulations can be applied to achieve a particular end result. In a preferred embodiment of the invention, the foam is applied in a series of very thin layers with minimal overlap. For example, the foam could be applied in about two to ten layers, each about 1 to 10 mm thick, with an overlap of about 1 to 80 cm, preferably about 3 to 50 cm.

The resulting foam may remain entirely on the press fabric, extending to an extent of 90% or more above the fiber surface plane, or it may be partially embedded in the surface to an extent of about 50%, such that 50% remains above the surface. Alternatively, the foam may be primarily embedded in the press fabric, penetrating partially or completely into the press fabric. Each layer is dried. After the top layer is dried, the coated press fabric is cured, e.g., by drying in air at room temperature for a sufficient period of time or at elevated temperatures for a period of time of about 1 minute to 5 hours. The temperature and time for drying or curing will depend on the foam used, manufacturing conditions, and the like.

The following examples serve to illustrate the invention and are not to be understood as a limitation of the invention.

EXAMPLES example 1

A water-based polyurethane emulsion containing 40% urethane solids was prepared and the emulsion was then foamed at a blow ratio of 6:1. The resulting foam was used to coat a DURAVENT (TM) press fabric (available from Albany International Corp.) in several passes.

The air permeability was tested using a Frazer air permeability tester. The results are shown in the table below. TABLE I Sample Number of Coatings Thickness of Coats Applied Air Permeability cm³ s⊃min;¹/cm² (cubic feet per minute/square foot) after drying after curing

It should be noted that the permeability was not affected by the curing step. It is possible to continue adding layers of foam until the desired permeability is achieved.

Example 2

Water-based foamed urethanes have been considered as a replacement for 100% solids polyurethanes for many reasons, such as controlling overlap when coating continuous structures or when better predictability of void volume is required. As discussed below, overlap can be controlled relatively tightly. Those skilled in the art will recognize that "100% solids polyurethanes" are those polyurethanes that contain little or no solvent and are referred to as "high solids" or "100% solids" polyurethanes.

When coating a press fabric with foam in several passes, it was found that for the specific foam used, the data satisfied the following empirical equation:

In (air permeability) = In (original press fabric air permeability) - (AP + BP²)

where A and B are constants (but not the same for all materials) and P is the number of coating passes. This formula provides an indication of the extent to which overlapping coats result in a change in permeability. After several coatings, the slight change caused by the overlap would be expected to result in no deterioration in the sheet properties.

A coated press fabric was prepared by applying layers of water-based polyurethane foam. The measured air permeability, in the form of measured and calculated data, is given in the table below. TABLE II Air Permeability (cubic feet per min./square foot) Sample Number of Layers Determined Calculated * Control

Example 3

Laboratory tests were conducted using a polyurethane foam formed from a water-based emulsion of Permuthane, the foam being applied to a DURACOMB (TM) 5710 fabric and a DURAVENT (TM) press fabric. A relatively low foam blow ratio (a blow ratio of 2.7) was used and multiple layers were applied. Air permeabilities were measured after each pass. Each fabric specimen was run in duplicate and the data from both runs are shown in the table below. TABLE III Air Permeability cm³s⊃min;¹/cm² cubic feet per min./square foot) Sample Fabric Uncoated Coating DURACOMB DURAVENT * Control

The data show that reproducibility is good. It is interesting to note that the open structure 5710 fabric closed more than the DURACOMB fabric on each run, indicating that specific approaches are necessary for each type of fabric being coated.

Example 4

Two sets of fabric specimens, SCREEN TEX (available from Albany International Corp.) and 5710 fabric, were coated with a foam formed from BF Goodrich acrylic latex. The objective was to produce a series of specimens with air permeabilities of approximately 40, 60 and 80 cubic feet per minute/square foot. The results are shown in the table below. TABLE IV Sample Fabric Number of Layers Air Permeability cm³s⊃min;¹/cm² (cubic feet per minute/square foot) SCREEN TEX * Control

Those skilled in the art of press fabric manufacturing will recognize that the target values for each series were well achieved.

Example 5

Test runs were carried out on a pilot paper machine using a range of press fabrics to determine the effects on the sheet drainage and sheet printability properties of newsprint. A typical newsprint furnish was used. The press arrangement consisted of three separate presses each equipped with its own press fabric, these presses are generally referred to as a "twinver press". Four press fabrics were used and classified as coarse, medium, super smooth and coated.

The coated press fabric represented the medium fabric substructure and needle punched fiber material, but with a foam coating of urethane emulsion. The purpose was to test whether the coating of a coarser structure, in particular coarser needle punched fibre material for use in press fabrics without any loss of properties. It was hoped that some improvements would be seen.

The data obtained are shown in Figures 1 and 2. Figure 1 illustrates the data at a newsprint solids content after the last press using a slightly different fabric run take-off angle geometry. This increase or decrease in the contact time between the press fabric and the paper web determines the amount of "rewetting" or the amount of water that is reabsorbed by the paper web at the fabric/paper web interface after mechanical removal, i.e., after removal from the paper web by the fabric.

As can be seen, under normal running conditions, the middle press fabric had the highest web solids. The letter X is the condition measured for the foam coated fabric, which was not measured for all fabric configurations. As can be seen, the solids were as high as any press fabric tested.

As shown in Fig. 2, a gradation of "0" (zero) is the paper web surface smoothness obtained by pressing the paper web against a smooth granite press roll. The goal is to create textile structures that remain close to this "0" (zero) mark under operating conditions.

As can be seen in Fig. 1, no negative effects were observed during web dewatering. A significant improvement in web smoothness for the coated fabric was observed for the mid-fabric, and the coated fabric resulted in a web surface almost as smooth as the super smooth fabric, as can be seen from the data in Fig. 2.

It should be noted that the super smooth fabric, which had a very fine base weave, and fine needle punched fiber material (all three denier fibers) would cause significant operating problems on a papermaking machine due to the filling, compaction, and wear of the three denier surface fiber. Web following, where the web does not exit the fabric cleanly after the press nip, would also be expected. During evaluation, none of these tendencies were observed with the coated fabric.

Further laboratory data obtained from three tests confirms that for a pressure sensitive furnish such as newsprint, the fabric-related smoothness increases are a result of the increased surface contact at the interface between the paper web and the press fabric. It follows, therefore, that the improved web smoothness values achieved were due to the increased contact area of the foamed press fabric compared to a fabric with a normal textile fiber surface.

Studies on handmade paper webs have long confirmed that porous, uniform surfaces with a high percent contact area exhibit greater web dewatering by mechanical action under controlled pressure printing conditions. Many studies published in the relevant literature on dewatering confirm this. There is still debate among proponents of either school of thought as to whether this effect is due to reduced rewetting in the nip or after the nip, or due to higher web dewatering in the nip. Regardless of which mechanism prevails, the porous foamed surface printing media disclosed above, with their higher surface contact area, controlled porosity and void volume, satisfy both of the above theories.

The specific embodiments described above serve to illustrate the implementation of the invention.

Claims (8)

1. A method of coating a press fabric for a papermaking machine to improve the water removal properties, the method comprising the following steps: (a) applying a thin continuous layer of a polymeric foam to the upper surface of a press fabric; (b) drying the foam; (c) repeating steps (a) and (b) once or more times sufficient to form an effective coating on the press fabric; and (d) Hardening the press fabric coated in this way. 2. The method according to claim 1, wherein the polymeric foam used is primarily a polyurethane foam. 3. The method according to claim 1, wherein the polymeric foam used is primarily a polyacrylic foam. 4. The method of claim 1, wherein the polymeric foam comprises one or more resin materials selected from the group consisting of polyurethanes, polyacrylates, polyethers, polyesters, polysilicones, polyvinyl chlorides, polyisocyanates, and polyacrylonitrile rubbers. 5. A method according to any one of claims 1 to 4, wherein the foam remains primarily on the surface of the fabric. 6. A method according to any one of claims 1 to 4, wherein the foam remains partially on the fabric surface and is partially embedded in the surface. 7. A method according to any one of claims 1 to 4, wherein the foam remains embedded beneath the surface of the fabric. 8. A method according to any one of the preceding claims, wherein the fabric material can be a woven fabric, a nonwoven fabric with or without needled fibers or a combination of several fiber configurations.